The present invention relates to a method of manufacturing an image sensor used for facsimiles, scanners, and the like, and more particularly to a method of readily manufacturing a reliable image sensor with thin film transistor switching elements.
In conventional image sensors, particularly a contact type image sensor, image information of a document, for example, are projected onto the sensor in one-to-one correspondence, and then the sensor converts the image information into electrical signals. In the conventional image sensor, a TFT drive image sensor is arranged such that the projected image information are divided into a number of picture elements (photo detect elements), charges generated by the photo detect elements are temporarily stored in load capacitors by using thin film transistor switching elements (TFTs), and then are time sequentially read out of the load capacitors in the form of electrical signals at a speed within a range between several hundreds kHz and several MHz. In the TFT drive image sensor, image data can be read out by a single drive IC, through the operations of the TFTs. Accordingly, the number of drive ICs for driving the image sensor may be reduced.
An equivalent circuit of the TFT drive image sensor is as shown in FIG. 6. As shown, it comprises a linear array 11 consisting of photo detect elements and having the length substantially equal to the width of a document, a charge transfer section 12 including a plurality of thin film transistors T.sub.N,n, which are provided in association with the photo detect elements 11' in one-to-one correspondence, and a multi-layer wiring section 13.
The photo detect element array 11 consists of N number of blocks of photo detect elements. One block consists of "n" photo detect elements and may be equivalently expressed by a photo diode PD.sub.N,n. The photo detect elements 11' are connected to the drain electrodes of the thin film transistors T.sub.N,n. The source electrodes of the transistors T.sub.N,n are connected, for each block, through the multi-layer wiring section 13 in a matrix fashion, to common signal lines 14 ("n" number of lines) and load capacitors C.sub.n. The gate electrodes of the transistors T.sub.N,n are coupled with a gate pulse generator (not shown), so that those are made conductive for each block. Charges generated by each photo detect element 11' are stored in a parasitic capacitance of the photo detect element and an overlap capacitor between the drain and gate of the thin film transistor for a predetermined period of time, and are sequentially transferred for each block to the load capacitor C.sub.n by using the thin film transistor T.sub.N,n as a transfer switch. To be more specific, in response to a gate pulse .phi.G1 from the gate pulse generator, the thin film transistors T.sub.1,1 to T.sub.1,n of the first block are turned on, the charges as generated and stored by the photo detect elements 11' of the first block are transferred to the load capacitors C.sub.n and stored therein. Potentials of the common signal lines 14 vary depending on the charges stored in the load capacitors C.sub.n. The voltage values are time sequentially picked up onto an output line 16 by sequentially turning on analog switches SW.sub.1 to SW.sub.n within a drive IC 15. In response to gate pulses .phi.G2 to .phi.Gn, the thin film transistors T.sub.2,1 to T.sub.2,n to T.sub.N,1 to T.sub.N,n of the second to N-th blocks are sequentially turned on, so that charges from the photo detect elements are transferred for each block. In this way, by sequentially reading the charges, image signals of one line in the main scan direction are obtained. A document is moved by an original feed means, such as a roller (not shown). A sequence of the operations is repeated, and finally image signals of the whole original are obtained (see Japanese Patent Unexamined Publication Nos. 63-9358 and 63-6772).
A specific structure of the conventional thin film transistor switching element (TFT) is as shown in FIG. 7. As seen, the structure of it is of the inverse stagger type. As shown, a chromium layer as a gate electrode 25, silicon nitride film as a gate insulating layer 26, hydrogenated amorphous silicon (a-Si:H) layer as a semiconductor active layer 27, silicon nitride film as a top insulating layer 29 disposed facing the gate electrode 25, and an n' hydrogenated amorphous silicon (n.sup.+ a-Si:H) layer as an ohmic contact layer 28 are layered on a substrate 21 successively from bottom to top. The ohmic contact layer 28 is divided into a part 28a of a drain electrode and a part 28b of a source electrode. An aluminum layer 30 as a wire is formed over the structure thus arranged. The drain electrode and the source electrode are an n.sup.+ hydrogenated amorphous silicon (n.sup.+ a - Si : H) layer and an aluminum layer, respectively. A wire led from an individual electrode of the photo detect element is connected to the part 28a of the drain electrode.
A method of manufacturing the thin film transistor switching element (TFT) follows. To form the gate electrode, chromium is vapor deposited over the substrate 21 and is delineated in a predetermined pattern. A silicon nitride film, gate insulating layer 26, of the gate electrode 25 is formed as an insulating layer. Hydrogen amorphous silicon (a-Si:H) as the semiconductor active layer 27 is deposited over the gate insulating layer 26 by the plasma CVD (Chemical Vapor Deposition) method. Succeedingly, a silicon nitride layer as the top insulating layer 29 is formed. The top insulating layer 29 is patterned. Then, n.sup.+ hydrogenated amorphous silicon (n.sup.+ a-Si:H) as the ohmic contact layer 28 is deposited by the plasma CVD method. Then, it is etched to form the part 28a of the drain electrode and the part 28b of the source electrode. Aluminum is vapor deposited over those parts 28a and 28b of the ohmic contact layer, and is delineated so as to shape a wire 30a led from the photo detect element 11', a wire 30b connecting to the top insulating layer 29, and a portion covering the top insulating layer 29. In this manner, the thin film transistor switching element (TFT) is formed.
With such a structure of the thin film transistor switching elements (TFTs) in the image sensor, however, the part 28a of the drain electrode and the part 28b of the source electrode are possibly damaged when aluminum is vapor deposited directly on the n.sup.+ hydrogenated amorphous silicon (n.sup.+ a - Si : H) layers as those parts 28a and 28b, or deposited thereon by the sputtering method. When those parts are damaged, the characteristic of the n.sup.+ hydrogenated amorphous silicon (n.sup.+ a - Si : H) is deteriorated so that the performance of the TFT and the reliability of the image sensor are deteriorated.
Additionally, in manufacturing image sensors, the photo detect elements 11' and the TFTs are formed in different process steps. Accordingly, the manufacturing process is complicated.